explicitly disable building from opam

Summary:
public
Building from opam is currently broken because opam doesn't preserve the
symlink structure that is needed by Infer. Fixing this from infer side is not
trivial and may have to wait until we have a working `make install` target.

Reviewed By: jberdine

Differential Revision: D2764478

fb-gh-sync-id: 8d53ef0

opam

@@ -8,9 +8,8 @@ bug-reports: "https://github.com/facebook/infer/issues"
 dev-repo: "https://github.com/facebook/infer.git"
 license: "BSD"
 build: [
-  ["./autogen.sh"]
+  ["echo" "building from opam is not supported yet"]
-  ["./configure"]
+  ["exit" "1"]
-  [make "infer" "java"]
 ]
 install: []
 remove: []